Polyethylene (PE) is categorized into different classes, based mostly on density and branching, for example high density polyethylene (HDPE), medium density polyethylene (MDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), very low density polyethylene (VLDPE), ultra high molecular weight polyethylene (UHMWPE), and ultra low molecular weight polyethylene (ULMWPE or PE-WAX). All of these are essentially linear except for LDPE, which has a branched structure.
Properties of PE generally depend on variables such as the weight-average molecular weight (MW), molecular weight distribution (MWD=MW/Mn), and the extent and type of branching (via crystallinity or densities). For example, Mw and MWD control PE melt processability. Low MW leads to a low melt viscosity, resulting in fast processing and less energy consumption. However, low MW tends to decrease mechanical properties, so applications which rely on good mechanical properties, such as film blowing, typically require polymers with bimodal MWDs.
LDPE is traditionally the highest volume resin for blown films. However, an important feature in the film blowing process is its ability to generate a high degree of drawdown so that the gauge of the final film is much thinner than the die gap. However, this is limited by the strength of the melt in the bubble. If the nip roll speed is too high, the tensile stress in the bubble will exceed the cohesive strength of the melt, resulting in a rupture of the blown film.
LLDPE, which has similar low amount of crystallinity as LDPE, began to replace LDPE in the 1980s. LLDPE, having lower melt strength but higher drawn speed to break than LDPE, may be more easily drawn down than LDPE. Therefore, a thinner LLDPE film can be blown with the same strength as the LDPE film or a LLDPE film is stronger having the same thickness as the LDPE film.
However, there are some problems in the film blowing process of LLDPE. Most LLDPEs have higher MWs but narrower MWDs. Also, their short chain branching (SCB) does not provide shear thinning. Traditional LLDPEs contain no long chain branching (LCB). This, in combination with their narrow MWDs increases entanglement densities, which produces highly viscous melts that require lower processing speeds. Therefore, LLDPE is more viscous at the high shear rates when processed in the extruder and the die than LDPE, which contains a certain amount of LCB. The more viscous LLDPE will result in increased screw torque, barrel wear, and melt temperature, and more frequent occurrence of undesirable sharkskin at the die exit. At the same time, LLDPE shows lower extensional stresses at low strain rates occurring in the molten tube and bubble inflation regions, rendering a higher chance of bubble instability. The narrow MWDs of mLLDPEs (LLDPEs made using a metallocene catalyst) make them even more difficult to process than conventional LLDPEs.
Accordingly, the blown film industry continues to experience a need for polyethylene materials with balanced melt strengths and drawn speeds to break, which also exhibit shear thinning.